Recycling of paper is taking on increasing importance as demands on resources increase. One of the aspects of recycling that is particularly challenging is the area of deinking. Various aspects of deinking are described in the literature. See, for example, L. D. Ferguson, "Deinking Chemistry: Part 1", TAPPI Journal, pages 75-83 (July 1992), and "Deinking Chemistry: Part 2", TAPPI Journal, pages 49-58 (August 1992); B. R. Read, "The Chemistry of Flotation Deinking", TAPPI Proceedings, 1991 Pulping Conference, pages 851-856; J. Jossinet, "Custom Deinking of Newsprint", 77th Annual Meeting Technical Section Canadian Pulp & Paper Association, pages 369-372 (1991); L. D. Ferguson, "The Role of Pulper Chemistry in Deinking", TAPPI Proceedings, 1991 Pulping Conference, pages 793-799; T. W. Woodward, "Appropriate Chemical Additives Are Key to Improved Deinking Operations", Pulp & Paper, pages 59-63 (November, 1986); and T. W. Woodward, "Deinking Chemistry", 1991 Chemical Processing Aids Short Course, pages 85-105. Washing deinking and flotation deinking are the two major types of deinking processes. Some preference is given to flotation deinking because it uses less water.
One of the greatest challenges in the area of flotation technology is the increasing amount of wastepaper which has been printed with water-based inks. The switch to water-based inks has been due, in large part, to the concern over the environmental impact of oil-based inks and the desire to reduce or eliminate the use of solvents in printing. As one will appreciate, the separation of oil-based inks from pulp during a recycling process in a water based environment is easier than the separation of water-based inks from water based pulp. The two greatest problems in using flotation for flexographic inks are: (a) the particle size is small after repulping, at 0.2-1.0.mu., and (b) the ink is hydrophilic and likes to remain with the water. (See discussion in the August, 1992 Ferguson article listed above at page 53-54.) Additionally, the systems that have been used by recyclers for oil-based ink separation are not always immediately adaptable to separations involving water-based inks. Contamination of regular furnish by water-based inks has caused significant problems by degrading the performance of deinking systems.
Various approaches have been tried to adapt flotation processes to flexographic inks. These approaches have included searching for chemicals that give improved results. Such chemicals have included collectors to remove the ink that has been released from the fiber (including fatty acids such as stearic acid), surfactants, displectors (combination of dispersants and collectors), water hardeners, and chelates.
Unlike oil-based ink, water-based ink has a water soluble polymer which carries and binds the ink to the substrate. A typical flexographic (water-based) ink formulation for newsprint may be on the order of 15% pigment, 70% water, 10% acrylic polymer, and 5% amines, defoamers, and other components. When the paper is pulped, these polymers (such as acrylic polymers) dissolve in the water, carry the ink throughout the pulp suspension, and redeposit onto the pulp fibers.
Flotation is not always useful in treating pulps containing water-based inks. Washing steps, even multiple washing steps are also not entirely satisfactory in removing residue from the pulp. For example, the gray water (the filtered water from the washing or thickening) may be recycled to the system and the resulting pulp is frequently darker than desired.
Bleaching steps have been used in conjunction with the deinking of flexographic materials to achieve brighter pulps.
Thus, an object of the present invention is to provide an improved process for deinking pulp which comes from sources containing flexographic (water-based) ink. It is a further object of this invention to provide a deinking process and deinking chemicals for flexographic ink which allows the use, in part, of flotation. These and other objects of the invention will be apparent from the following description.